1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method.
2. Discussion of the Related Art
Conventionally, an apparatus such as a printer to form a color image using a tandem structure has been incorporating a technology to correct positional errors in order to match positions of respective colors of the image. For example, JP-2005-292760-A discloses a color image forming apparatus in which, during continuous printing of plural sheets of paper, positional error correction marks are transferred to a conveyer belt or to an intermediate transfer body by lengthening conveyance intervals between successive recording sheets. The advantage of the color image forming apparatus disclosed in JP-2005-292760-A is that it can perform positional error correction during continuous printing operation without stopping the apparatus.
As is known, two transfer methods are available in the image forming apparatus: an intermediate transfer method and a direct transfer method. The intermediate transfer has the advantage of creating fewer positional errors between transferred colors compared to the direct transfer. By contrast, the direct transfer, if used in the image formation of fewer colors of toner, such as monochrome image formation or two-color image formation, can reduce the time taken to output media on which such less-than-full-color images are formed.
In addition, JP-2008-90092-A discloses an invention related to an image forming apparatus configured to include both the intermediate transfer body and the direct transfer body. Specifically, the same discloses a technology to set the time that the image transferred to the intermediate transfer body reaches the position for the direct transfer to be an integral multiple of the time taken for one cycle of rotation of the intermediate transfer body, thereby reducing the positional error of the transferred image due to deviation in the rotation speed of the rotational body.
However, the same does not consider scaling errors in a sub-scanning direction. In the structure including the both intermediate transfer and the direct transfer, the speed of the intermediate transfer and that for conveying a paper medium at the side of the direct transfer are not necessarily the same. Then, this difference in speed causes sub-scanning scaling errors.
For these reasons, a need exists for an image forming apparatus including both the intermediate transfer body and the direct transfer body capable of solving the aforementioned conventional problems of the sub-scanning scaling errors while providing both high-speed printing of the less-than-full-color color images and high-quality multicolor image printing collaterally.